The present invention relates to an etch-resistant photoresist system, particularly for light in the deep ultraviolet range, and a method for producing dimensionally accurate structures, particularly in the sub-.mu.m, on semiconductor substrates using the photoresist system of the present invention.
Plasma etching is one of the most important process steps in manufacturing structures for micro-electronics. Substrates that have a surface composed of semiconductors, insulating material, or of metals, can be structured through plasma etching. In many plasma etching processes halogen-containing plasma is used. The plasma is composed, for example, of etching gases such as CF.sub.4, SF.sub.6, BCl.sub.3, Cl.sub.2, CHCl.sub.3, CHF.sub.3, perfluoropropane, and hexafluoroethane. An example of such an etching process is the etching of a silicon substrate with CHF.sub.3 or CF.sub.4 gas.
During the etching process, in order to produce a structure in the silicon substrate, the surface regions that are not to be etched must be covered with an etch-resistant material. The etch-resistant material must in turn be structured, in a lithographic manner, with radiation. Radiation, such as visible or ultraviolet light, gamma rays, or electron rays are typically used in this regard to modify the chemical structure of the etch resist such that, for example, it becomes less or more soluble to a developer in comparison to the unirradiated regions. As a result thereof, the more soluble regions of the etch-resistant layer dissolve out, the structure of the more insoluble regions remain. These regions must have an adequate stability in the etching plasma used for etching the substrate.
Principally organic polymers are utilized as resists. Resists that predominantly contain aromatic hydrocarbons have a significantly higher stability vis-a-vis a halogen-containing plasma than do resist having a purely aliphatic structure.
In addition to the resist-associated considerations such as structurability and etching resistance, additional considerations must be taken into account for producing a highresolution structure. In a photolithographic process, for example, the properties of the stepper used for the exposure or, of the stepper lens that influence the obtainable minimum structural size that is obtainable (critical dimension CD) must be considered as well as wavelength and the numerical aperture NA pursuant to the equation: ##EQU1##
Because standard photoresists are structured through exposure to light having wavelengths of 365 nm or 436 nm, the resolution of the photoresist can be improved, according to the equation set forth above, by utilizing a shorter exposure wavelength, for example 248 nm. However, alkaline developable etch-resistant resists that produce half-.mu.m structures, having an adequately layer thickness (.gtoreq.900 nm) do not exist for this wavelength; this wavelength (248 nm), is in the deep ultraviolet range.
Most commercially available resists (for example, a resist based on a novolak), due to their aromatic structure, have a highly unbleachable absorption particularly in the deep ultraviolet range (DUV) which includes, for example, light having a wavelength of 248 nm. Therefore, these resists are unsuitable for producing fine structures with DUV. In this regard a highresolution resist must have good bleachability. Additionally, a resist should initially have a high absorption in the range of exposure in order to guarantee a correspondingly high sensitivity of the resist. However, during the course of the exposure, the resist should become transparent with respect to the radiation utilized, so that the layer regions of the photoresist that lie therebelow can also be reached by an adequately great light intensity.
Heretofore, this problem has been dealt with in that a DUV-transparent material was photo-structured and an etching resistance was then only produced in the photoresist structures thereafter on the basis of a suitable treatment.
In published patent application GB-A 2 121 197, it is disclosed that a positive electron resist polymethacryloylchloride is after-treated with aromatic amines and silicon or aluminum compounds in an organic solution after lithographic structuring in order to increase etching resistance. This method, however, includes the disadvantages that the resist material is sensitive to hydrolysis and forms corrosive hydrochloride acid with moisture that cannot be avoided. Further, the method suffers the disadvantage that the reactions are extremely difficult to control.
EP-A 00 98 922 discloses a bilayer system wherein the ultraviolet absorption of the upper resist layer is increased through treatment with vapors of naphthalene, anthracene, and the like. The etching resistance of the upper resist layer is thus increased. But, simultaneously therewith, the absorption of wavelengths below 300 nm is also increased. The resist layer is thereby barely sensitive at all to these wavelengths. The process is also so complex that it is unsuitable for commercial production.
U.S. Pat. No. 4,289,573 discloses a method for increasing the etching resistance of resists by treatment with sodium hydroxide or potassium hydroxide. The method, however, is also unsuitable for commercial production purposes since alkali ions are completely undesirable in silicon technology. Moreover, this process is extremely complex to implement.